Many medium and high volume copiers and printers such as electrophotographic printers employ sheet media transport systems that use air flowing through holes in a plenum adjacent to the sheet media to control the sheet media. One example is the document feeder shown in U.S. Pat. No. 4,421,306 issued Dec. 20, 1983 to Muka.
Air movers such as fans and blowers having a rotating impeller (hereinafter referred to simply as fans) used to provide air pressure to such sheet media handling apparatus tend to be large and noisy. Because large fans cannot be fitted easily within the media path, ducts are used to direct air flow to the plenums used for media transport. The use of the ducts causes aerodynamic losses which necessitate the fans be even larger and/or run at a higher speed and hence noisier. A second problem occurs when the apparatus is employed at high altitudes, such as in Denver, Colo. Obtaining adequate performance of the sheet media handling apparatus in a lower density air at high altitudes requires a higher flow rate which in turn requires a higher fan speed. A machine designed to perform adequately at the highest elevation that may be encountered will be unnecessarily noisy at lower elevations where the extra air flow is not required. Consequently, it is the current practice to manually adjust the speed of the fan for installation of equipment at high altitudes. It is the object of the present invention to solve these shortcomings in the prior art sheet media handling apparatus.